Asphalt has been used as a construction material by man since at least as early as biblical times. The slime pits of Gomorrah, used in mortar in the construction of the tower of Babel were asphalt. In more recent times, asphalt has been used as a cement in the construction of pavement. Asphalt emulsions are widely used in the manufacture and maintenance of pavements. One problem encountered is that the hot or warm asphalt emulsion is too fluid and may separate from the aggregate. This is particularly true if the asphalt emulsion has been modified with a latex of a synthetic polymer. The modification of asphalt with synthetic polymers has been well known since at least the 1930's. There are a number of texts discussing the application and modification of asphalt including Asphalts and Allied Substances, Fifth Ed., by Herbert Abraham, Van Nostrand; Asphalt Science and Technology, Edwin J. Barth, Gordon and Breach; Asphalts and Road Materials, Modern Technology, John E. Parson, Noyes Data Corp., 1977; and A Basic Asphalt Emulsion Manual, The Asphalt Institute Manual Series No. 19 (MS-19), March 1979.
Applicant has been able to locate only one patent which discusses the use of various natural thickeners in asphalt emulsions.
British Pat. No. 437,674, published Nov. 4, 1935, in the name of E. I. Du Pont de Nemours and Company discloses a dispersing agent for aqueous emulsions of asphalt. The dispersing agent is an alkali (including ammonium) or alkaline earth salt of an alkyl sulphuric acid, the alkyl radical containing at least 8 carbon atoms. The disclosure also states "The emulsions may also contain thickening agents such as casein, albumen, glue, gelatin, resin, or starches, tannin, gums, dextrin and tapioca flour . . . ." Cationic rapid set emulsions typically have a pH of about 3. Conventional anionic emulsions have a pH of about 12. Generally, unmodified starches degrade in a relatively short time at pH's of less than about 5 or greater than about 10.
At the present time, there are no convenient post-additives which may be subsequently added to an existing asphalt emulsion to built and maintain its viscosity. Generally, if the viscosity of an asphalt emulsion is too low, it is necessary to blend the emulsion with an emulsion having a higher viscosity to try to raise the viscosity of the asphalt emulsion.